This invention is in the general field of multi-ply tissue products as well as methods and apparatus for bonding tissue plies to make such products.
Various tissue products are made by laminating multiple layers of tissue. These layers or plies must be bonded to prevent them from delaminating or floating apart when they are converted into the final product or when they are used by the consumer.
Ply bonding of the individual tissue sheets into multi-ply layers is usually achieved by embossing them on converting equipment without applying adhesive. In this process, two or more tissue webs are simultaneously unwound and fed through a nip formed between male and female embossing rolls to emboss or crimp the webs and thereby bond them together. Often in making such products as napkins, the webs are embossed only around the perimeter of areas that will be cut into the individual napkins.
In a different process, tissue plies may be adhered using a chemical adhesive rather than by embossing. For example, Bjork U.S. Pat. No. 5,466,318 discloses a process for laminating webs using a water based adhesive.
Controlling ply bonding is important and difficult. Inadequate, excessive or inconsistent ply bonding can jam complex, high-speed machinery, generate high waste, and provide unacceptable product. The strength of bonding by embossing (without adhesive) may vary depending, among other things, on water content or dryness of tissue webs, and on ambient air humidity. While bonding by embossing sometimes can be improved by increasing the pressure on the embossing rolls, such pressure can wear out the embossing rolls more quickly, particularly the female roll, which is usually a softer roll made of composite material onto which a pattern is impressed by the opposing, male, engraved metal roll. In addition, the journals and bearings of both embossing rolls can also wear out prematurely if subjected to increased pressure over a prolonged period.
The use of adhesives can avoid some of the problems of embossing, but adhesives also cause problems, such as "through bonding" or "blocking", in which adjacent laminates bond to one another, and, prevent unwinding of the laminate product from its roll. Non-uniformity of bonding also causes problems, such as wrinkling of the tissue and bad printing.
As noted, the bonding process may be part of the overall process for making the final multi-ply product. For example, multi-ply tissue products, particularly napkins, may be printed, e.g., by a letterpress or a flexographic process. In a typical letterpress printing process, a multi-ply web is first embossed, and then the central unembossed area of individual napkins is printed. In an alternative flexographic printing process, unbonded webs may be first run through a flexographic printing tower and then crimped with an embossing nip.
While the invention has broader application, we refer specifically to certain markets, particularly high quality napkins such as the party goods, food service, and other specialty napkin markets. These markets may require cloth-like character and feel, high wet strength, and the ability to be decorated with high quality printing. Increasingly, these markets have expressed preference for quality and complexity provided by flexographic printing.
It can be difficult to achieve cloth-like character and feel while also achieving satisfactory bonding of two or more plies of tissue to make one sheet of napkin tissue that will not come apart either when used by the consumer or when subjected to flexographic printing. The napkin must not deply, it must stay in one piece like a cloth napkin would. However, the use of adhesives and other additives to develop the resistance to ply separation tends to make the product stiff and destroy the cloth-like feel.
Another product requirement in this field is the need of strength while wet, which can be critically important to high quality paper napkins. Non-wet strength napkins may pull apart when subjected to moisture, such as spilled liquid or a wet glass, rendering then unsatisfactory for the high quality markets. Chemicals normally employed to impart wet strength (e.g., urea, phenol-formaldehyde) can reduce ply bond-strength which is already problematic, and can make the napkin stiffer.
With regard to decorated or printed napkins, certain napkin machines (e.g., those made by Servotec and Hoberna) which use flexographic printing have stringent ply requirements. As noted, these flexographic machines print the napkin first and emboss it afterwards, in contrast with letterpress napkin machines, which first emboss the edges of the napkin so that the plies of tissue are crimped together, after which the napkin is printed. In the flexographic process, it is critical that the plies be held together without embossing while the napkin is being printed. Floating plies as the tissue enters the printing stage result in jams and high waste together with machine downtime. In addition, the application of the ink causes the top layer of the tissue to expand if the plies are not well bonded, resulting in wrinkling, badly printed napkins, and extremely high costs from waste.
These problems are sometimes addressed by the use of much heavier napkin tissue (as has been done in Europe where some of the flexographic machines were introduced). Heavier tissue is more expensive.